Production of sodium hydroxide and chlorine from brine constitutes an important part of the electrolytic industry. Many improvements have been made on the process and electrolytic cell in this field.
The most advanced industrial process in this field is the ion exchange membrane process. It employs an insoluble anode having an overvoltage as low as tens of mV and an activated cathode having an overvoltage of about 100 mV, between which is interposed an ion exchange membrane whose electric resistance has been owing to recent improvements. Therefore, the electrolytic voltage of this process is about 3 V, which is close to the theoretical value of 2.2-2.4 V. In other words, this process has reached a stage in which there is no room for further improvement in energy saving (except for unavoidable ohmic loss).
The electrolysis of sodium chloride is represented by the following chemical equation. EQU 2NaCl+2H.sub.2 O.fwdarw.2NaOH+Cl.sub.2 +H.sub.2 ( 1)
Although the sodium hydroxide and chlorine gas as the products are fully utilized, the hydrogen as the by-product is not fully utilized at the present time. The electrolytic voltage required for the evolution of hydrogen is theoretically about 0.83 V. This voltage is equivalent to the amount of power consumption that would be saved if an adequate measure is taken to carry out electrolysis without the evolution of hydrogen.
The means developed for this purpose is the oxygen depolarizing electrode (gas electrode), which is based on the principle that if the cathode is supplied with oxygen gas, the cathode reaction proceeds as shown below EQU H.sub.2 O+2e+1/20.sub.2 .fwdarw.20H.sup.- (0.40 V) (2)
in place of the conventional reaction represented by EQU H.sub.2 O+e.fwdarw.OH.sup.- +1/2H.sub.2 (-0.83V) (3)
The theoretical consequence is the saving of electric power equivalent to about 1.2 V.
Hydrogen peroxide is an oxidizing agent used for pulp bleaching, etc. There is an established technique for producing hydrogen peroxide from oxygen or oxygen-containing gas (such as air), also using a gas electrode. Its improvement is still going on, as reported in "Denki Kagaku" (Electrochemistry), 58, 11, 1073, 1989, for example.
The gas cathode itself is known as disclosed in Japanese Patent Publication No. 29757/1990 and Japanese Patent Laid-Open No. 25179/1983. With the gas electrode, it is possible to lower the voltage by about 0.8-1.0 V. It has a hydrophobic porous layer on one side thereof and a hydrophilic layer carrying an electrolytic catalyst on the other side thereof. The catalyst may instead be formed on said hydrophobic porous layer. The catalyst can be electrically conductive carbon carrying platinum thereon, for example.
The gas electrode, however, has the disadvantage that although it works satisfactorily in the initial stage of electrolysis, it loses its catalytic activity in a short period of time because it is in direct contact with concentrated alkali hydroxide (the electrolyte in the production of hydrogen peroxide, for example) during electrolysis. Moreover, it is very difficult to produce a gas electrode of large area which prevents the gas and liquid from penetrating into each other. Therefore, no gas electrode has ever been put to practical use for large-scale industrial electrolysis. Another disadvantage of the gas electrode is that if air is used as the gas, the membrane becomes clogged with sodium carbonate resulting from the reaction of alkali hydroxide with carbon dioxide contained in air. This poses a problem associated with the removal of carbon dioxide from air to be used as the gas.
An idea of providing the gas electrode with an ion exchange membrane, thereby causing it to supply H.sup.+ and/or OH.sup.- to the electrolyte compartment, has been proposed in U.S. Pat. No. 3,124,520. Although this idea seems favorable to a large-sized gas electrode, it has never been put to practical use because no details are known about the conditions of actual use.
The above-cited literature describes an electrolytic cell for production of hydrogen peroxide which is divided into an anode compartment, intermediate compartment, and cathode compartment by a cation exchange membrane and anion exchange membrane, with the anode compartment accommodating a gas cathode. A disadvantage of this electrolytic cell is that the gas cathode is not in close contact with the anion exchange membrane and, hence, comes into contact with the catholyte (which is a corrosive aqueous solution of potassium hydroxide). This shortens the life of the gas cathode.
The gas electrode for electrolysis of alkali chloride, on the other hand, is expected to be put to practical use in the near future for the purpose of saving energy. However, it still has a problem associated with its life.